Various wireless networks use an estimated received signal strength indicator (RSSI) and an estimated carrier to interference and noise ratio (CINR) of a received signal to determine operational characteristics of the networks. For example, IEEE 802.16e compliant mobile stations (MSs) are required to estimate an RSSI and a CINR of a received signal. In general, CINR at an MS may be calculated as the ratio of an RSSI of a serving base station (BS) to summed RSSIs of interferer (non-serving) BSs plus a white noise power of a receiver of the MS. The RSSI associated with a serving BS has been used by an MS for uplink power control and the CINR, which has been reported to a serving BS, has been used by the serving BS to adapt a downlink transmission rate to link conditions. The CINR of a signal received from a non-serving BS (e.g., a potential handover target) may also need to be estimated and monitored by the MS to, for example, compare with the CINR of a current serving BS to determine whether a handover should be triggered. In general, any BS of interest, for which a CINR is to be estimated, may be referred to as a “target” BS.
Accurate reported CINRs are desirable, because inaccurate reported CINRs may impact performance of a cellular network. That is, reporting a CINR that is above an actual CINR may decrease network throughput due to re-transmission required after unsuccessful attempts, while reporting a CINR that is below the actual CINR may cause the serving BS to schedule data rates below a supportable rate. As another example, to identify potential handover candidates, an MS has also been required to track RSSIs and CINRs associated with non-serving BSs. Inaccurate RSSIs/CINR may trigger late or unnecessary handover. RSSI and CINR estimation is often determined from a specific pilot signal transmitted from one or more BSs. As an example, according to IEEE 802.16e, RSSI and CINR estimates are derived based on a preamble signal, which is an orthogonal frequency division multiple access (OFDMA) symbol that is transmitted at the beginning of each OFDMA frame. The preamble signals transmitted from BSs are unique to each BS, but the preamble signals can still interfere with each other due to non-orthogonal preamble sequences.
A number of techniques have been proposed to estimate CINR. For example, U.S. Patent Application Publication No. 2006/0133260 discloses a channel-estimation based-approach that isolates noise and interference components using pilot sequences and estimates a true channel power by subtracting a combined noise and interference power estimate from a total received power estimate. While this approach usually functions adequately for noise-limited systems (i.e., system where noise dominates interference), due to the non-orthogonality of the pilot sequences for serving and non-serving base stations (BSs), the CINR estimates provided by the approach may include an undesirable fixed bias in interference-limited systems (i.e., systems where interference dominates noise). As another example, U.S. Patent Application Publication No. 2006/0093074 discloses a difference-based approach for estimating CINR that assumes that sub-channels on adjacent pilot locations are the same. Based on this assumption, noise and interference components are isolated by subtracting adjacent received signals. However, assuming that sub-channels on adjacent pilot locations are the same may result in a fixed bias that is not insignificant compared with noise power at moderate to relatively high CINRs (e.g., CINRs of 15 decibel (dB) or higher).
Furthermore, neither of the above-described techniques distinguishes between interference and noise powers, i.e., both techniques combine interference power and noise power. In IEEE 802.16e compliant systems, RSSI and CINR estimates are based on a preamble signal, whose preamble signal power is boosted with respect to data signal power. While noise power is the same for the preamble signal and a data signal, signal and interference powers of the preamble signal are boosted as compared to the data signal. However, IEEE 802.16e requires reported RSSI and CINR to reflect a data signal power in a data zone (i.e., compensating for the power boosting in preamble). As such, accurate CINR estimates for a data zone are difficult to obtain for techniques that obtain a single estimate for combined noise and interference power. Additionally, at relatively low CINRs, e.g., 15 dB or less, errors in combined interference and noise power estimates may introduce large errors in RSSI.
What is needed are techniques for accurately determining received strength signal indicator and carrier to interference and noise ratio estimates for received signals.